The United Nations aid chief warned Monday that Syria is the greatest humanitarian crisis of our time and there “is a race against time” to save hundreds of thousands of besieged civilians with food, water and medical supplies.

“In Aleppo, we risk seeing a humanitarian catastrophe unparalleled in the over five years of bloodshed and carnage in the Syrian conflict,” Stephen O’Brien told the U.N. Security Council in his monthly briefing on the situation.

“Once again," he added, "I cannot stress strongly enough the need for a 48-hour pause in fighting to be approved by all sides and come into effect, so that safe and sustained humanitarian access is opened to all areas of Aleppo.”

Since last month, the U.N. has been asking for weekly 48-hour pauses in the fighting in Aleppo in order to get aid in and the sick and injured out.

O’Brien said the U.N. is ready to move some 70 trucks with aid into eastern Aleppo as soon as it receives the necessary security guarantees. The eastern part of the city is held by opposition forces, but has been encircled and under siege by government forces for weeks, effectively trapping some 250,000 people.

The government controls the city’s west, where more than a million people still reside. Aleppo is Syria’s largest city and before the civil war began, the country’s commercial center.

O’Brien said the humanitarian crisis goes well beyond Aleppo. Despite having asked the Syrian government to authorize aid deliveries to 32 locations across the country in August, he said the government had denied the U.N. access to more than 50 percent of requested beneficiaries. In addition, fighting and administrative delays have contributed to not a single convoy moving so far this month.

The aid chief told the Security Council — which has faced a political divide over Syria — that he is “very angry” and that the “callous carnage” has long since moved from “the cynical to the sinful.”

“Now is the moment, this instant, to put differences aside, come together as one and stop this humanitarian shame upon us all, once and for all,” O’Brien urged.

Russia’s U.N. envoy, Vitaly Churkin, told the council that “the government is doing everything to ensure that the civilian population has a more or less normal life; they are doing this despite unilateral economic sanctions.”

Churkin said work is under way to define the parameters of the pauses, and he warned that fighters seeking to resupply and regroup should not exploit them.

Last week, Russia announced its support for the U.N.-proposed 48-hour weekly humanitarian pauses in Aleppo.

“We need to see Russia follow its unilateral declaration with genuine steps to support regular and sustained access to Aleppo,” U.S. Deputy U.N. Ambassador Michele Sison told the council.

The United States supports the pauses. Sison said they are necessary because the people of Aleppo “need U.N. help now,” but that a more comprehensive arrangement that revives the collapsed nationwide cessation of hostilities is needed to guarantee wider aid access.